Catalyst deactivation due to carbon formation is one of the most difficult challenges in the design of catalysts for the reforming of hydrocarbon fuels. Carbon deposits decrease catalyst activity by blocking active sites, causing attrition of catalyst particles and results in increasing pressure drop and ultimately discontinuation of the process.
The formation of carbon on the catalyst initiates either by reaction of the hydrocarbon on the active metallic surface or by cracking on the support material. Carbon formation on metallic surfaces is a structure sensitive reaction that preferentially initiates on the step edges of the metal crystallites. While step edges are more reactive toward C—C and C—H bond scission, their surface geometry results in highly coordinated sites (a greater number of next-nearest-neighbors) that facilitate the multipoint adsorption of heavy hydrocarbon molecules and the formation of carbon. By dispersing the active metals in a mixed-metal oxide that has a strong metal-support interaction; such as hexaalumina, the active metals are maintained in a state of low coordination, with few contiguous clusters; thereby, disrupting the carbon formation mechanism. Ensembles, or groups of active sites, are also nucleation sites for carbon growth. By dispersing the active metal crystallites, the nucleation sites are eliminated minimizing carbon deposition. In this invention, transition metals are doped directly into the structure of hexaalumina, a high temperature refractory support material. The hexaalumina serves to not only disperse the active metal crystallites, but also creates a strong metal-support interaction that prevents these crystallites from aggregating, sintering and vaporizing.